Influence of nonlinear noise correlation on transmission range

It has been established that the physical mechanism for increasing the operating range of coherent communication systems with digital compensation for linear signal distortions in the receiver is to reduce the correlation of nonlinear interference noise from neighboring spans in the absence of physical chromatic dispersion compensators. It is shown that changing the correlation coefficient $\varepsilon$ from 1 (a characteristic value for a fiber-optic link with full physical chromatic dispersion compensation) to 0 (the minimum value for a fiber-optic link without physical chromatic dispersion compensation) leads to a several times increase in the operating range. The optimal relationship between the gains of erbium fiber amplifiers and optical power losses in spans adjacent to the amplifier has been determined, ensuring maximum operating range of lines with an arbitrary value of the nonlinear noise correlation coefficient.